He was captured on video using an ATM in Beltsville, later at a McDonald's in Burtonsville and then arriving in a taxi at The Mall in Columbia on Jan. 25.

Nineteen-year-old Darion Marcus Aguilar also left another tip for investigators by uploading a "selfie" taken inside the store's dressing room moments before he started shooting, creating one final freeze-frame marking the precise time of his attack.

Howard County police were not able to trace Aguilar's exact steps in the hours before he opened fire in the mall, killing two clothing store clerks, but surveillance video from around the region helped them account for his actions from the time he left home in College Park at 5:15 a.m. and fired his first shots at 11:14 a.m.

Cameras aren't exactly everywhere — they captured Aguilar riding escalators in the mall but missed him as police say he waited near the food court before the shooting — but the range of footage released last week showed just how broad their coverage has become.

Civil liberties groups, which have fought against the expansion of public surveillance, have used the proliferation of private cameras — in security systems, on smartphones and in vehicles — to argue against government-run programs.

Jay Stanley, a senior policy analyst for the American Civil Liberties Union Speech, Privacy & Technology Project, wrote in a recent blog post that such "Little Brother" systems mean "we can get 99% of the security … with a much smaller portion of the privacy downside."

One big caveat for the ACLU: the systems shouldn't be placed under any type of centralized government control.

"When a freak event such as a terrorist attack or more common crime such as a mugging takes place, the police will be able to collect video footage from the private citizens and businesses that are nearby," Stanley wrote.

Baltimore County State's Attorney Scott D. Shellenberger cited a homicide at Towson Town Center mall, in which investigators, using the facility's 268 surveillance cameras, were able to piece together footage of the suspects trailing the victim through the mall.

"Surveillance cameras now are an integral part of every serious prosecution that we proceed on," he said